Due to the high absorption of radiation at used wavelengths in the EUV wavelength range between approximately 1 nm and approximately 35 nm, no refractive optics, such as for example lens elements, but rather mirror elements are typically used as optical elements for this wavelength range. Such optical elements which reflect EUV radiation absorb a portion of the EUV radiation, which is incident on the optical surface during operation, and expand in the process. Due to the absorption, or the associated expansion, deformations occur on the optical surface of these mirror elements, which result in undesired optical aberrations.
WO 2012/013747 A1 discloses controlling the location-dependent temperature distribution in a substrate of a reflective optical element using a temperature control device in two or three spatial directions to correct aberrations. The temperature control device can have heating elements, for example in the form of resistance heating elements which can be arranged in a grid. It is also possible for radiation sources which act on the substrate or on the reflective optical element by way of thermal radiation (for example IR radiation) so as to thermally influence it to be provided as heating elements. An absorption layer serving for absorption of the IR radiation can here be arranged below a reflective surface of the optical element. In order to produce a temperature distribution in the substrate which is as homogeneous as possible, it is possible for the radiation sources to be configured for supplying thermal radiation onto the front side of the substrate at which the EUV radiation is reflected, or for supplying thermal radiation to the rear side of the substrate.
WO 2009/152959 A1 discloses a projection exposure apparatus for semiconductor lithography, having a device for thermally manipulating an optical element which has a front side for reflecting electromagnetic radiation and a rear side. Provided are thermal actuators what act on the optical element from the rear side. The thermal actuators can be LEDs or lasers, the emission spectrum of which can be in the IR wavelength range. Such thermal actuators can emit electromagnetic radiation which at least partially passes through the substrate and which is at least partially absorbed by an absorption layer which is disposed on the front side of the substrate on which a multi-layer coating is also applied. A coating exhibiting high absorption for radiation emitted by the actuators can also be applied on the rear side of a plane mirror. A substrate which is transparent for radiation at the used wavelength can be disposed on the front side of the plane mirror.
For the purposes of heat dissipation, optical elements reflecting EUV radiation are typically cooled from the rear side and/or from the peripheral surfaces. Due to issues relating to installation space, the heat sinks used for this purpose frequently cannot be designed in an ideal fashion and generate a non-constant, location-dependently varying temperature distribution on the rear side of such an optical element. In principle, it is possible, with sufficient installation space, for the temperature distribution in a substrate to be appropriately set or thermally homogenized tomographically in all three spatial directions.
For example, it is known from WO 2013/044936 A1 to arrange a wavefront correction apparatus having a refractive optical element in a microlithographic projection lens. A first and second partial region of a circumferential peripheral surface of the refractive optical element can be respectively irradiated with first and second thermal radiation which at least partially penetrates the optical element. A refractive index distribution within the optical element, caused by the partial absorption of the thermal radiation, serves for changing, or at least partially correcting, a wavefront error.
PCT/EP2013/000728 discloses the arrangement of a wavefront correction apparatus in the form of a mirror having a reflective coating and a mirror substrate in a projection lens. A first and second partial region of a circumferential peripheral surface of the mirror substrate can be respectively irradiated with first and second thermal radiation which at least partially penetrates the mirror substrate. A temperature distribution in the substrate, caused by the partial absorption of the thermal radiation, results in a deformation of the mirror which serves for changing, or at least partially correcting, a wavefront error.
In order to neutralize the thermal profile caused by the heat sink or to homogenize the temperature distribution in the substrate, additional heating from the front side of the substrate, for example using a radiation source or using the resistance heating elements which were described further above, can be effected in principle. However, a coating reflecting EUV radiation, which is disposed on the front side of the substrate, could be damaged by the additional introduction of heat and that hysteresis can occur, for example, if the thermal profile on the rear side of the substrate is intended to be set or regulated by the action on the front side of the substrate.